1. Field of the Invention
The present invention relates generally to sterilization of vessels, and more specifically to the sterilization of fermentation vessels used for the production of aqueous solutions of ethanol.
2. Description of the Prior Art
Aqueous solutions of ethyl alcohol, or ethanol, are produced by microorganisms, such as yeast, from sugars derived from such crops as sugar cane or by enzymatic transformation of starch or cellulose in biomass to form sugars for alcoholic fermentation. In the latter fermentation processes, starch-containing crops or cellulosic biomass, which contains lignin, cellulose, and hemicellulose, is first broken down in water by enzymatic or acid hydrolysis into sugars, such as glucose or xylose. Yeast microorganisms then consume these sugars, converting them into an aliphatic alcohol, such as ethanol. In industrial alcohol production facilities, this entire process, or at least its final stages, occur in large fermentation vessels.
If these fermentation vessels are not properly disinfected or sterilized between batches or uses, bacteria and other undesirable microorganisms can become attached to the interior walls of the fermentation vats where they will grow and flourish. These undesirable microorganisms may contaminate ethanol co-products such as animal feed, or they may consume valuable quantities of the substrate, or sugar, thus reducing the production of ethanol. The economics and efficiency of fermentation processes are frequently such that they cannot tolerate any such loss of production.
Current methods used to kill these unwanted microorganisms often involve introduction of foreign agents, such as antibiotics, heat, and strong chemical disinfectants, to the fermentation before or during production of ethanol. The addition of each of these foreign agents to the process significantly adds to the time and costs of ethanol production. Antibiotics are very expensive and can add greatly to the costs of a large-scale production. The use of heat requires substantial energy to heat the fermentation vessels as well as possibly requiring the use of special, pressure-rated vessels that can withstand the high temperatures and pressures generated in such heat sterilizing processes. Chemical treatments can also add to the cost of production due primarily to the cost of the chemicals themselves and secondarily to the fact that these chemicals are often hazardous materials requiring special handling and environmental and safety precautions.
While the efforts in the ethanol production industry to sterilize and disinfect fermentation vessels have been focused almost exclusively on the use of such costly and hazardous chemicals, as described above, there has been very little thought given or efforts directed to an effective and efficient use of ethanol itself as a disinfectant in the ethanol production process, even though the effectiveness of ethanol as a disinfectant for other purposes has been long-known. For example, U.S. Pat. No. 903,853, issued to Gartner back in 1908, described the use of a heated aqueous solution of ethanol or methanol to disinfect books, including a solution of 100 parts 96% alcohol and 80 parts water. U.S. Pat. No. 3,908,031, issued to Wistreich et al. on Sep. 23, 1975, similarly describes using ethanol in the vapor phase to sterilize food products and spices, including a concentration of ethanol in water that was at least 80% by volume and at least 78.degree. C., but preferably 150.degree. C. in order to prevent condensation of the ethanol vapor on the food products or spices.
Heden, in his U.S. Pat. No. 3,997,400, describes the use of unheated, concentrated alcohol in conjunction with a strong chemical, such as betapropiolactone, to sterilize the interior walls of the tanks of an oil tanker so that they might be used as fermentation vessels to produce yeast fodder for cattle feed when the tanks are not being used to haul oil.
Tegtmeier, in his U.S. Pat. No. 4,845,033, attempted to take advantage of the somewhat higher alcohol, pH, and temperature tolerance of yeast over some other microorganisms to minimize such other microorganisms in a continuous alcohol production process. Tegtmeier used a two-stage continuous process to do so. In his first stage, Tegtmeier provides a good nutritional, oxygen, and growing environment for desirable alcohol producing yeast microorganisms to get a healthy population and good cell sizes of those desirable microorganisms for use in his second stage to produce alcohol while controlling pH, oxygen, and temperature operating parameters to discourage undesirable microorganisms. Tegtmeier also keeps the alcohol concentration in his second stage higher than most undesirable microorganisms can tolerate. Therefore, undesirable microorganisms produced in the first stage and injected along with the desirable yeast microorganisms into the second stage have difficulty thriving in the second stage. Unfortunately, the desirable yeast microorganisms, while surviving, are stressed and not as vigorous or healthy in that high alcohol concentration either, so they do not perform as well as they could in a lower alcohol concentration, and alcohol production suffers. However, Tegtmeier's process accepts this trade-off of reduced production capability of the desirable alcohol producing microorganisms for the benefit of reducing the undesirable microorganism population in his second or production stage.
Consequently, there is still much to be desired in the field of ethanol production, particularly in batch production processes, for effective fermentation vessel sterilization that is also safe, low cost, and environmentally sound, yet which enhances, rather than degrades or limits efficient alcohol producing microorganism activity.